There is a need to be able to accurately and precisely continuously meter small quantities of powdered materials, for example 1 to 9 micrograms per second. The electronics industry has a need to meter small quantities of powdered materials to a vaporization zone for direct vapor deposition or for precursors in chemical vapor deposition (CVD). There is also a need to be able to accurately and precisely meter material amounts three orders of magnitude higher, for example 1000 micrograms per second. In many systems, it would be advantageous to be able to meter powdered materials over the range of 1 to 1000 micrograms with the same equipment. Organic light emitting diode devices (OLEDs), for instance have a light emitting layer that often contains a host and a dopant that are deposited in amounts differing by two to three orders of magnitude. There would be a great advantage in OLED manufacturing to be able to independently, and continuously, meter powdered organic materials to a vaporization region using a common transport design for host, co-host and dopant materials.
It is well known that precisely metering small amounts of powdered materials is difficult. There are numerous examples of systems that make use additional materials as carriers and additives to facilitate the transport of powdered materials. Carriers that have been used include inert gases, liquids, and solids. The use of any sort of additive increases the material transport complexity, for the carrier or additive needs to be added, removed and handled separately from the actual material of interest. The use of carriers also increases the risk of contamination, which is particularly detrimental in the pharmaceutical and electronics manufacturing industries where there is a particular need to meter materials.
In U.S. Pat. No. 3,754,529, Fleischner describes an auger device for transporting powdered material that has been mixed with an inert carrier, preferably sand. The ratio of active material to sand is reported to be 1:9. Transporting a mixture that is mostly inert carrier adds costs and complexities to the system, and increases the potential for the introduction of contaminates into the material feed.
Commonly assigned U.S. Patent Application Publication Nos. 2006/0062918 and No. 2006/0177576 use a traditional auger design to meter powders, where there is a patterned screw within a smooth barrel. FIG. 1 shows a cross-sectional view of a typical prior art auger structure showing a patterned auger screw 5 within a smooth barrel 7. Auger screw 5 of an auger structure 8 is turned by a motor (not shown). The distance between the threads of the screw helix and the thread height are chosen to be sufficiently large that powder tends not to pack into and rotate with the helix, but rather to remain at the bottom of the horizontally oriented auger barrel 7 and is transported linearly by virtue of the relative motion between auger screw 5 and auger barrel 7. In the horizontal orientation as shown, the powdered material travels primarily along the bottom of auger screw 5 in a tumbling and dispersed form. The terminal end of auger screw 5 can be configured to have a thread-free portion 9 as shown having a constant circular cross section over a small length to constrain the consolidated powder to form a narrow annular or tubular shape. One of the problems with using this type of auger structure with powders is varying discharge rate. The discharge rate has been observed to vary cyclically with the angular orientation of auger screw 5. The quantity of material discharged by the auger from revolution to revolution is quite reproducible, but within a revolution it is quite variable. In the horizontal orientation, more powder resides in the lower half of the auger barrel than resides in the upper half and this can accentuate the cyclic discharge. Using the auger in a vertical orientation such that the powder is evenly distributed around the interior of the auger barrel can attenuate the cyclic discharge, but the cyclical variation remains and the mechanical drive arrangement for the auger and agitator is more complicated.
The metering device of this disclosure can also be used as one part of a larger vapor deposition system. Vapor deposition systems of particular interest are those designed for manufacturing organic light emitting diode (OLED) devices. An OLED device includes a substrate, an anode, a hole-transporting layer made of an organic compound, an organic luminescent layer with suitable dopants, an organic electron-transporting layer, and a cathode. OLED devices are attractive because of their low driving voltage, high luminance, wide-angle viewing and capability for full-color flat emission displays. Tang et al. described this multilayer OLED device in their U.S. Pat. Nos. 4,769,292 and 4,885,211.
Physical vapor deposition in a vacuum environment is the principal way of depositing thin organic material films as used in small molecule OLED devices. Such methods are well known, for example Barr in U.S. Pat. No. 2,447,789 and Tanabe et al. in EP 0 982 411. The organic materials used in the manufacture of OLED devices are often subject to degradation when maintained at or near the desired rate-dependent vaporization temperature for extended periods of time. Exposure of sensitive organic materials to higher temperatures can cause changes in the structure of the molecules and associated changes in material properties.
To overcome the thermal sensitivity of these materials, only small quantities of organic materials have been loaded in sources and they are heated as little as possible. In this manner, the material is consumed before it has reached the temperature exposure threshold to cause significant degradation. The limitations with this practice are that the available vaporization rate is very low due to the limitation on heater temperature, and the operation time of the source is very short due to the small quantity of material present in the source. In the prior art, it has been necessary to vent the deposition chamber, disassemble and clean the vapor source, refill the source, reestablish vacuum in the deposition chamber and degas the just-introduced organic material over several hours before resuming operation. The low deposition rate and the frequent and time-consuming process associated with recharging a source has placed substantial limitations on the throughput of OLED manufacturing facilities.
A secondary consequence of heating the entire organic material charge to roughly the same temperature is that it is impractical to mix additional organic materials, such as dopants, with a host material unless the vaporization behavior and vapor pressure of the dopant is very close to that of the host material. Additionally, the standard use of separate sources creates a gradient effect in the deposited film where the material in the source closest to the advancing substrate is over-represented in the initial film immediately adjacent the substrate while the material in the last source is over represented in the final film surface. This gradient co-deposition is unavoidable in prior art sources where a single material is vaporized from each of multiple sources directly onto a substrate. The gradient in the deposited film is especially evident when the contribution of either of the end sources is more than a few percent of the central source, such as when a co-host is used. FIG. 2 shows a cross-sectional view of such a prior-art vaporization device 10, which includes three individual sources 11, 12, and 13 for vaporizing organic material. Vapor plume 14 is preferably homogeneous in the materials from the different sources, but in fact varies in composition from side to side resulting in a non-homogeneous coating on substrate 15.
Commonly assigned U.S. Patent Application Publication Nos. 2006/0062918 and No. 2006/0062919 overcome many of the shortcomings of the use of separate point sources by metering materials to a flash vaporization zone. U.S. Patent Application Publication No. 2006/0062918 teaches the metering of host and dopant mixtures in a single powder transport mechanism, and using a manifold to distribute the vapor to the substrate. U.S. Patent Application Publication No. 2006/062919 discloses the ability to mix organic vapors in the manifold and deliver a mixture of materials to the substrate surface. However, none of these earlier teachings anticipate the need to have independent metering control for the host and dopant materials. The transport mechanisms are therefore unable, by virtue of design, to meter at the low rates, 1-10 micrograms/second, required for an independent dopant feed.
U.S. Patent Application Publication Nos. 2007/0084700 and 2006/0157322, U.S. Pat. Nos. 6,832,887 and 7,044,288 disclose powder feeding pumps for moving powders from an entry port to a discharge port using parallel spaced disks that rotate within a housing having an internal cavity that defines a volume having an increasing volume from the input port to the discharge port. These powder feeding pumps are intended for use with much larger particle size powders and are not adapted to metering powder on a milligram or microgram basis.
There continues to be a need to precisely control the metering of milligram to microgram quantities of powdered material into a vaporization apparatus.